California State University Sacramento -- Department of Civil Engineering
CE 135: Hydraulics Laboratory
Flow Over a Sharp-Crested Weir -- PROCEDURES
Recommended Reading:
Crowe, C.T., D.F. Elger and J.A. Roberson Engineering Fluid Mechanics, 8th ed., John Wiley
and Sons, 2005, pp. 555-559
Crowe, C.T., D.F. Elger and J.A. Roberson Engineering Fluid Mechanics, 7th ed., John Wiley
and Sons, 2001, pp. 607-611
References:
1. Brater, E.F. and H.W. King, Handbook of Hydraulics, 6th ed., McGraw-Hill, Inc., 1976
2. Daugherty, R., J. Franzini, and E. J. Finnemore, Fluid Mechanics with Engineering
Applications, 8th ed., McGraw-Hill, Inc., 1985.
Objectives:
1. To examine the fundamental characteristics of flow over two sharp-crested weirs
2. To plot flow profiles for vented and unvented weirs
3. To measure the coefficient discharge values of a rectangular and triangular weir
4. To gather pressure information for a future lab on hydrostatic forces
Apparatus:
1. One-half meter flume and sharp-crested weir with manometer board
2. V-notch weir in return-flow channel
3. Water surface gage
4. Manometer board
Experimental Procedure:
1. Set up for the experiment.
a. Open the surge tank valve and turn on the large pump.
b. Close the drain valve on the flume head tank and open the gate on the downstream end of
the flume.
c. Open the feed valve and fill the head tank until it overflows the weir. Operate the system
until you see overflow from the V-notch weir in the sump.
d. Close the feed valve and then the downstream gate on the flume. You should have
several inches of water in the flume. Check that all the piezometers attached to taps
downstream of the weir are reading the same level. If not, bleed air from the tubing.
When you are satisfied that the air is out, drain the flume by opening the downstream
gate. Leave this gate open for the rest of the lab. When the water just covers the flume
bottom, read the water levels in the piezometers and record them on the “Water Surface
Profiles” data sheet. These tell you the elevations of the taps downstream of the weir.
e. Check that the water height is at the weir crest elevation. The weir leaks a little, so you
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may have to introduce a little additional water to bring the static water level to the height
of the weir. The weir piezometers should be at the same level as the water upstream of
the weir. If not, bleed air from the tubing. Use the point gages to measure the static
water elevations at the weir crest height and the bottom of the flume. Do the same for the
V-notch weir. Measure the elevations of water surface at the vertex of the V-notch and
the bottom of the return channel.
2. Measure the pressure on the weir at one medium flow rate (for use in a later lab report):
a. With the static water level at the height of the weir, record the level of each weir face
piezometer column (those labeled with letters).
b. Measure the distance of each pressure tap on the weir face from the top of the weir using
a tape measure (or copy the numbers from the face of the weir itself).
c. Establish the flow over the weir crest necessary to obtain a head, H of about 0.3 ft (0.09
m). Record the flowrate from the in-line flow meters. Make sure the weir vent is open.
Record the level of each weir face piezometer column.
d. Put the weir pressure data away in a safe place. You will use it in a future lab.
3. Measure the flow profile over the rectangular weir at the same flow rate as in part 2.
a. Establish the flow over the weir crest necessary to obtain a head, H of about 0.3 ft (0.09
m). Determine the flowrate from the in-line flow meters. Make sure the weir vent is
open.
b. Record all the piezometer levels.
c. Record point gage readings of the water surface above each piezometer tap and as far out
along the drawdown zone as possible.
d. Repeat steps b and c with the vent closed. Wait for awhile after closing the vent to
establish a truly unvented weir profile.
g. Open the valve on the ventilation pipe at the base of the weir and note the change in the
nappe profile and level of water backed up against the downstream face of the weir.
Close the vent and observe the same things. Open the vent.
4. Calibrate the rectangular and V-notch weirs.
a. Establish a steady flow and record the flowrate. Using the upstream point gage, measure
and record the water levels (head) at the rectangular weir (ventilated and unventilated)
and at the V-notch weir. Note: Storage of water in the channel may cause the V-notch
weir head to stabilize slowly, so don’t record the height prematurely.
b. Repeat the previous step at least five more times.
5. Shut-down
a. Close the flume feed valve. Open the drain valve on the head tank.
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b. Turn off the pump and close the surge tank valve.
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